I'm on my third all-grain batch using a simple bucket system (Williams Brewing Mash Kit). In my all-grain beers am repeatedly getting this late aroma on the finish that is unpleasant, very strong and lingering. I can only describe it as

What is the flavor of the beer like? Have all of these AG beers had a similar flavor that you suspect is related to the "hay" aromatics?

I'm not sure, but to guess, I'd say you are lautering too aggressively or are not vorlaufing well or long enough. This can allow husk fractions to be transferred into the wort to be boiled. The other thought I have is that you are not getting a good enough boil to volatilize some of the rough grain aromatics. If it is the first, you should be able to detect the flavor of tannins (astringent). If the beer is tannic, mashing/vorlauf/runoff is the source and you are transferring the problems to the kettle.

Give me a complete rundown of the grains including brands and age and your mash/vorlauf/runoff proceedures including temperatures and times and I can probably figure it out.

This is the recipe for the worst one. I was trying to make a very low alcohol bitter. During bottling the hay-like aroma was almost overwhelming.
4 lbs. Maris Otter
2 lbs. Crystal 40 L
Single infusion Mash
158 deg F for 60 min, 1.8 qts/lb using bottled drinking water (very soft) supplemented with a tablespoon of gypsum and dash of Epsom salts. Vorlauf: Ran 0.5 gal. very slowly and returned to grain bed. Drained to Brew pot and fly sparged with 2 gal 170 deg F water. Topped off with bottled water to 6 gal.

After reading your reply, I can't fault anything but the Maris. I used to use around 45,000 to 55,000 pounds of Maris a year, bith from Simpsons and Crisp. Since I buy direct from the wholesalers, you'd assume it would be in the best condition. I found that it was inconsistent at best and poor at worst. It had a wide variability in extract and at times imparted stale grain flavors into my baches. Oxidized grains can leave a flavor reminiscent of "grassy". So, I'd guess it is the malt. Easy to test this... brew with another base malt. If the notes are gone, bingo!

It is alot. But it is also difficult to get flavor when you only go for 3% alcohol. The beer does taste good until the finish. As I mentioned, the huge % of crystal could be the origin of the flaw. However, as Mesa suggests, it may very well be a bad batch of Maris Otter. Some people I have talked with say they had the same problem with MO they used about the time I brewed the beer.

As Mesa suggested, I brewed with a new batch of Maris Otter. Made the Autumn Apple Spiced Ale in the recent issue of BYO. Also vigorously pre-boiled for 30 min. The sweet wort is delicious; no hint of the strange grassy aroma/flavor. Thanks for the help.